This invention relates to conduits for transmitting hazardous fluids and more particularly to long length transmission conduit which includes a surrounding conduit for safety purposes.
Dual conduit apparatus has been utilized in the past for various purposes including the transmission of hazardous gases and the like. Typically, the inner conduit is arranged to transport the hazardous fluid while the outer conduit provides a backup or a means to monitor the integrity of the primary line or even as a device for counteracting primary line leakage by means of inert fluids or the like. In such systems the inner conduit may be connected for transmission of fluid at a low level of pressure while the outer conduit may be connected to a higher pressure source. In the event of leakage in the inner conduit, flow then would occur from the outer to the inner conduit and the change in pressure of the outer conduit, for example, could be monitored to detect the failure.
Some hazardous fluids are extremely dangerous but may be required to be routed through a manufacturing facility for great distances in order to isolate or secure the source of fluid and yet provide the material at various locations where it is required in a manufacturing process. In the past it has been the practice to provide a redundant, tube within a tube-type system by fabricating the assembly from relatively short lengths of rigid conduit. Such conduit in small diameters may be bendable to facilitate routing through turns, but in the larger diameters of the outer conduit, this may be difficult or impractical. Some prior art installations currently employ rigid threaded pipe for the outer conduit and while this provides a primary advantage of strength and rigidity for the system, it also includes the inherent drawbacks of inflexibility, sensitivity to thermal expansion, multiple interconnecting joints, relatively large size and high cost, and difficulty of installation and maintenance. While many of these factors can be tolerated the bottom line is that the multiplicity of joints in the system create a condition which is inherently sensitive to environmental influences and very dependent upon the quality of the initial installation. In the transmission of hazardous and toxic fluids, such conditions are difficult to control even with the redundant conduit systems currently being employed in industry.
Other forms of long length tubes are known in the art which are provided for the transmission of fluids under controlled conditions, most often including thermal barriers therein. One example of this is U.S. Pat. No. 4,194,536 which describes several examples of long length tubing products of various configurations, including single and plural side-by-side tubes, with or without heating cable or heat tubes and combined in a jacketed structure with thermal insulation. Such product is bendable and deformable for facility of routing but does not comprehend the tube within a tube structure of the instant invention.
Other prior art structures are described in U.S. Pat. Nos. 4,240,850 and 4,351,365. In both of these references, drawn, continuous inner tube of copper for example, is sheathed by various materials including a cover of plastic or metal strip which is longitudinally closed thereover. In both of these examples means are provided for concentrically spacing the inner and outer tubes, while the space therebetween is filled with thermal insulation material for controllihg the thermal characteristics of the composite structure. No free space is provided between the inner and outer tubes for an inerting fluid, for example.
One form of a rigid conduit system is disclosed in U.S. Pat. No. 3,693,648 and this is typical of cumbersome prior art structures.